Cook potatoes in large saucepan of boiling salted water until tender, about 20 minutes.
Meanwhile, in a saucepan over medium-low heat, warm the half-and-half, horseradish and nutmeg until small bubbles form around the edges of the pan.
Drain potatoes and mash while gradually adding the butter and the half-and-half mixture. Beat with a large spoon until the potatoes are smooth and creamy. Season with salt and pepper.